Towards rational design of nanoscale lubricants and elucidation of the hydration lubrication mechanism using molecular simulation

Abstract

Nanoscale devices featuring surfaces in sliding contact are found in applications ranging from biocompatible materials for joint replacement to the next generation of hard disk drive technologies. While several approaches to lubricating such systems have been proposed, as well as successfully implemented, we lack the ability to properly explore this design space. In this dissertation are described 1) a collection of tools designed to enable large scale screening of soft materials across chemical parameter space and 2) the application of these tools to studying surface functionalized materials for use in nanoscale lubrication systems. The core tool, mBuild, generates starting configurations for simulations by minimizing or even eliminating the need to explicitly translate and orient components when building systems - users simply state which components to connect. This approach enables users to programmatically vary parameters for a family of systems (e.g., polymer chain length) or interchange individual components (e.g., polymer type), while still employing the same general framework. mBuild integrates with the Foyer tool for cataloging and applying force fields to molecular systems. Foyer provides a force field and simulator agnostic method for defining parameter usage that relies upon SMARTS based annotations of chemical context, providing both human and machine readable documentation of parameter usage, which also aids in the dissemination of force fields. We demonstrate the utility and flexibility of our tool chain by screening the frictional properties of several families of monolayers. The tool chain is also employed to study the molecular origins of the experimentally hypothesized hydration lubrication mechanism as it is manifested in poly(2-methacryloyloxyethyl phosphorylcholine), or pMPC, based materials. Surface bound films of this biocompatible polymer have been shown experimentally to produce tribological properties that surpass those of the human synovial joint and are being developed for use in artificial joints. The mechanism by which such materials are thought to provide ultra-low friction coefficients has been termed “hydration lubrication” . The work presented herein lends strong support to the hypothesis that hydration lubrication is manifested in pMPC systems and additionally that the phosphorus bearing moiety is responsible for enabling this mechanism while the choline group may serve to stabilize the overall brush structure thus giving rise to the remarkable durability of the experimentally grown brush layers.